Sutures are considered joints based on structure and joints based on function.

Sutures are considered FIBROUS JOINT based on structure and SYNARTHROSES joint based on function.
In humans, joints are classified based on their structures and functions. Based on structures, joints are classified into three, these are fibrous, cartilaginous and synovial joints. The fibrous joints is made up of connective tissues which can not move, for instance, sutures.
Based on fuctions, the joints are divided into three, which are: synarthroses, amphiarthroses and diarthrosis. The synarthroses joint is made up of fibrous joints.